Organic EL elements, particularly, low molecular weight organic EL (hereinafter referred to as OLED), have been isolated function thereof by Eastman Kodak based on ultrathinning and multilayering of organic layers and are thus remarkably improved in characteristics such as drastic lowering of drive voltage (Applied Physics Letters, U.S.A., 1987, Vol. 51, pp. 913-915). Cambridge University found an EL element using a polymeric fluorescent material (hereinafter referred to as PLED) (Nature, England, 1990, Vol. 347, pp. 539-541), which has now been improved in characteristics to such a level as to compare with conventional OLED elements.
On the other hand, it was found that with OLED elements, provision of a copper phthalocyanine (CuPC) layer as a hole injection layer leads to an improvement in initial characteristics such as low drive voltage and high luminous efficiency and an effect of prolonging the life of the elements (Applied Physic Letters, U.S.A., 1996, Vol. 69, pp. 2160-2162). With PLED elements, it has been shown that similar effects are obtained using, as a hole transporting layer (buffer layer), polyaniline materials (Nature, England, 1992, Vol. 357, pp. 477-479, Applied Physics Letters, U.S.A., 1994, Vol. 64, pp. 1245-1247), and polythiophene materials (Applied Physics Letters, U.S.A., 1998, Vol. 72, pp. 2660-2662). At a cathode side, it has been found that initial characteristics could be improved using, as an electron injection layer, metal oxides (IEEE Transactions on Electron Devices, U.S.A., 1997, Vol. 44, pp. 1245-1248), metal halides (Applied Physics Letters, U.S.A., 1997, Vol. 70, pp. 152-154), and metal complexes (Japanese Journal of Applied Physics, 1999, Vol. 38, pp. L1348-1350). These electron layer and buffer layer have been now in general use.
However, CuPC that has been ordinarily used as a hole injection material in OLED element has the drawback that it has drastic irregularities and thus, brings about a great characteristic lowering upon mixing in other organic layers in small amount. Polyaniline-type materials and polythiophene-type materials currently employed for PLED element involve problems in that they contain, as a solvent, water having capability of promoting element deterioration, limitation is placed on the choice of solvent, limitation is also placed on the manner of coating ensuring uniform film formation because of coagulation of material and low solubility, and a difficulty is involved in control of viscosity.
Based on the facts set out hereinabove, organic solvent-based charge transporting varnishes using low molecular weight oligoaniline materials have been recently found. It has been found that insertion of a hole injection layer obtained by use of this type of material enables excellent EL element characteristics to be shown (see JP-A 2002-151272).
In forming a charge transporting thin film from an oligoaniline compound, however, long-time firing at high temperature in the presence of oxygen is usually necessary. Thus, where low molecular weight oligoaniline materials are used as a hole injection layer in OLED element or PLED element, a prolonged time is required for the fabrication of the element, so that a problem has been indicated in the lowering of productivity. Hence, there is a demand for shortage of a firing time after the film formation.